feat: on conditional disable, do not import additional plugin modules

[abeldekat]

Example:

{
    {
      "folke/tokyonight.nvim",
      lazy = true,
      opts = { style = "moon" },
    },

    {
      "LazyVim/LazyVim",
      priority = 10000,
      lazy = false,
      opts = {
        colorscheme = "tokyonight",
      },
      version = "*",
      import = "lazyvim.plugins",
      -- cond = false,
      enabled = false,
    },
}

In this scenario, the LazyVim plugin is disabled, and the modules in lazyvim.plugins are not loaded.
However, when using cond = false, the modules in lazyvim.plugins are loaded, disregarding the cond setting.

For the lazyflex plugin, it makes sense to enable/disable the plugin using cond.
In issue Simpify installation instructions the motivation for this PR is described.

This PR adds a check on cond to lazy.core.plugin, method Spec:import(spec).

I tested the changes using my lazy starter fork, see config.lazy.

[folke]

Thanks!